1. Field of the Invention
The present invention relates to a novel apparatus for recording and reproducing information and, more particularly, to an information recording and reproducing apparatus in which the recording, erasion and reproduction of information are made by making use of a change in the crystalline structure of the recording medium caused by application of light or heat energy to the recording medium.
2. Description of the Prior Art
In recent years, there is an increasing demand for higher densification and digitization of information recording, which in turn gives rise to the demand for development of various information recording/reproducing systems. In particular, optical disc, which is capable of recording, erasing and reproducing information in cooperation with a laser beam energy, exhibits a higher recording density than conventionally used magnetic discs and, hence, is considered quite promising for the future. The optical disc is described in detail in Industrial Rare Metal No. 80, 1983" (Optical Disc and Material), pp 37-41. Optical discs capable of reproducing information by laser beam have been put into practical use under the name of Compact Disc (abbreviation "CD").
On the other hand, the presently available systems which enable recording of information can be classified into two types: namely, an unerasable type and rewritable type. The unerasable type permits writing only once and does not allow erasure, while the rewritable type permits reapeated writing and erasure. The recording method of the unerasable type forms fine convexities and concavities in the medium by breaking or shaping the medium by means of a laser beam, while the reading of the recorded information is conducted by making use of a change in the light reflectance due to interference of the laser beam caused by the minute concavities and convexities. For instance, it is well known to form convexities and concavities by melting or sublimation of a recording medium made of, for example, tellurium (Te) or its alloy, by means of a laser beam. This type of recording medium, however, involves the problem of toxicity.
On the other hand, photoelectromagnetic materials are major materials for the rewritable type medium. The recording with this material is conducted by inverting local magnetic anisotropy in the medium at a temperature around the Curie point or compensation point by the energy of a light, while the reproduction is conducted by means of a rotation amount of plane of polarization caused by the magnetic Faraday effect or magnetic kerr effect brought about by the polarized incident light. This recording/reproducing method is considered as being the most promising method using the medium of rewritable type and an intensive study is being made with a prospect that this method will be put into practical use within several years. Unfortunately, however, no material has been found which would provide a sufficiently large rotation amount of the plane of polarization. At the present stage, it is still impossible to obtain sufficiently high level of output such as S/N (Single/Number) and C/N (Carrier/Number), despite various efforts such as lamination of the medium material.
Another known system of rewritable type makes use of a change in the reflectance caused by a reversible phase change between amorphous state and crystalline state of the recording medium material. Unerasable type mediums making use of unidirectional or irreversible phase change between crystalline state and amorphous state also have been known. These mediums, however, suffer from problems such as poor reliability of the disc due to lack of phase stability at room temperature, because the crystallization of the amorphous phase takes place at low temperatures.